The larger one is the leader class Energon Megatron. The smaller verison was a two-pack with Energon Optimus Prime (this was voyager sized). Hope that helps you out, bro. What custom do you plan on doing with it, if you don't me asking?

I had one of each left not long ago, but JAF picked them both up from me through that old sale thread of mine.

The only real color differences are the shoulder cannons being a darker "clear" green than the smaller version, as well as the landing gear on the legs (and some slight lighter colors to the orange on the legs). I believe the heads were painted a different color (just a lighter shade on the smaller one and some of the plastic was molded in a lighter shade). Overall, they look pretty damn similar. (I used the Voyager version for my Galvatron custom, for reference).

I already snagged one on ebay for $20...
like I mentioned, I have the headless one here, so I am going to mod up the legs and waist to give him about 2 more inches in height. The mods won't effect the alt mode (canon) either.